If I get the latest version of blender, will I lose anything in my old project files?
Blender is for the most part backwards compatible, that means files created with previous versions will generally open problem free in future versions.

Apart from major version jumps (like from 2.4# series to 2.5 or like the future 2.7# series to 2.8) Blender should open files created in older versions without any problems. Some times changes are made and "version patching" will happen internally automatically, that means blender will automatically try to fix anything that changed so it will look as close to original as possible. That being said there may be an occasional thing that breaks or causes a minor glitch, but never to the point of totally breaking or being unable to open your work.

Hell for the most part Blender even prides itself in being able open files created with recent versions of the software in very old versions, albeit with lots of warning and broken things.